1. Field
This application relates generally to gas turbine exhaust noise reduction.
2. Description of Related Art Including Information Disclosed Under 37 CFR 1.97 AND 1.98
The optimum cross sectional exit plane area of a gas turbine engine's converging-diverging, (supersonic) exhaust nozzle is a function of the nozzle throat area, exhaust gas speed, stagnation pressure, thermodynamic conditions, and other factors. Differences in pressure between the exhaust stream and ambient airflow can reduce the engine's efficiency, and increase radiated noise. For this reason, the gas turbine engines powering many high performance tactical aircraft are fitted with variable geometry exhaust nozzles that can expand or contract to provide an exit plane area optimized for efficient thrust delivery at most throttle settings, pressure altitudes, etc.
Although a properly contoured supersonic exhaust nozzle, e.g. a “de Laval” nozzle, may provide the most efficient generation of thrust for one set of operating conditions (exhaust speed, temperature, pressure, flight altitude, etc.), the wide range of tactical operating conditions requires the use of variable geometry exhaust nozzles. A de Laval nozzle operating at its design condition will perfectly expand the supersonic exhaust stream, i.e. there will be no shock waves in the exhaust plume, and therefore no “shock associated noise” or “screech”. Variable geometry nozzles, by the nature of their frustoconical convergent and divergent sections, operate off-condition, so that the exhaust plume is imperfectly expanded. At military power (non-afterburning) takeoff conditions for most tactical aircraft, the exhaust nozzles operate in an over-expanded condition, meaning that the pressure in the exhaust stream is below that of the ambient air stream. The shock waves generated by this imperfect expansion contribute to the supersonic jet noise generated by tactical aircraft exhaust flows. However, the dominant source of noise from tactical aircraft is the supersonic convection of large turbulent structures in the nozzle exhaust plume. One proven means of reducing the strength of this noise source is to increase the mixing between the high-energy jet exhaust and the low-energy ambient air. Doing so reduces the amount of energy available for noise generation in the exhaust plume, and also increases the frequency of the radiated sound. This is beneficial because the atmosphere attenuates higher frequencies much more rapidly. Mixing apparatuses, such as chevrons or tabs, may be added to the nozzle to increase mixing, but these devices reduce the engine thrust, increase complexity, and add weight penalties.